<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* .box{
      color: blue;
    }
    #app{
      color: red;
    } */

    /* #a{
      color: red;
    }
    .b{
      color: blue;
    }
    p{
      color: green;
    } */

    /* .app .box p{
      color: pink;
    } */
/* 
    .box > div {
      color: aquamarine;
      height: 30px;
    } */

    /* .one + .two{
      color: red;
    } */
     
    /* .one:hover{
      color: red;
    } */

    /* .one::after{
      content: 'hello';
    } */


    /* .two[data-index]{
      color: blue !important;
    } */
     
    /* #one{
      color: red !important;
    } */

/* 
    .box > div {
      height: 30px;
      background-color: aqua
    } */

    p + section{
      height: 30px;
    }

    section {
      height: 20px;
      background-color: aqua;
    }
    
  </style>
</head>
<body>
  <div id="app" class="app">
    <div class="box">

      <div class="one" id="one" >
        <p class="p">one</p>
        <section>two</section>
      </div>




      <!-- <div class="two" data-index="1">two1</div>
      <div class="two">two2</div>
      <div class="two">two3</div> -->



<!-- 
      <div id="a" class="b">
        hello
        <div>css</div>
      </div>
      <div>word</div> -->

    </div>
  </div>
</body>
</html>